The present invention relates generally to measuring the physical properties of objects or fluids and, more particularly, to methods and sensors for determining the physical properties of objects or fluids in multi-path clutter environments. For the purposes of defining and describing the invention, it is noted that a “fluid” should be taken to define any matter with fluidic properties including, but not limited to, a gas, a liquid, a vapor, etc.
It is often necessary to measure physical properties such as temperature, strain, pressure, etc. using a wireless system. In some cases, there are a large number of multiple reflections (multi-path signal propagation environment) of the radio signals along the propagation path so that the signal to or from the sensor will be corrupted and/or modulated by the multi-path environment. When there is a number of varying coherent signal reflections along the propagation path, the result is multi-path induced variations in the phase, amplitude, and/or time domain character of the signal. This situation presents a very serious problem for sensor system design. Also, practical constraints on sensor placement, weight, size, temperature, and lifetime requirements present problems to engineers in the design of very small and light weight sensors that can operate wirelessly without a source of power.
For the purposes of describing and defining the present invention, it is noted that the term “multi-path clutter environment” refers to an environment in which electromagnetic waves are transmitted and received in the presence of reflecting structures. These reflecting structures may be stationary or may be moving within or through the environment. The reflecting structures are capable of reflecting the electromagnetic waves such that an electromagnetic wave sent through this environment may be reflected off one or more reflecting structures before reaching its intended destination. As a result of these reflections, the amplitude, phase, and/or time delay of a transmitted electromagnetic wave may be altered by the time it reaches its intended destination. Furthermore, since the reflecting structures may be moving within or through the environment, the particular reflections experienced by individual electromagnetic waves may vary in an unpredictable manner.
For the purposes of describing and defining the present invention, it is noted that the term “radio frequency signal” is an electromagnetic wave having a frequency of 1 Megahertz or higher. The term “radio frequency” may be abbreviated as “RF.”
For the purposes of describing and defining the present invention, it is noted that the term “ringdown” is utilized herein to refer to the process of the energy decay of an RF signal in a multi-path clutter environment. Similarly, “multi-path ringdown time” refers to the time required for the multiple reflections of a RF signal in a multi-path environment to decay to a low enough value to be statistically or empirically insignificant. For example, the multi-path ringdown time may be defined as the amount of time necessary for the reflected RF signals to decay to an amplitude of 10% or less of the original RF interrogation signal. Other means of defining this time may be used, as is known in the art.
For the purposes of describing and defining the present invention, the term “wireless sensor” refers generally to a sensor which is capable of measuring a physical property of an object or fluid and transmitting information relating to that measurement to another device without the use of conductors or “wires.” The wireless sensor itself, however, may have conductors or wires which are used to internally connect the individual components of the sensor. Furthermore, the wireless sensor may lack an internal power source, such as a battery. Instead, the energy necessary to perform a measurement may be wirelessly transmitted to the wireless sensor via electromagnetic waves (e.g., RF signals).